The invention relates to an embossing device for tissue paper comprising a first and a second roll cooperating with each other to emboss a web of tissue paper in-between.
The embossing of sheet materials or the deforming of sheet or strip material, especially of non-woven fibre webs, is usually carried out by means of two rotatable calender rolls having an embossing nip formed in-between. The web material to be embossed is guided between these rolls. At least one of the cooperating rolls is provided with raised discrete locations forming a signature surface pattern to be embossed. These raised discrete locations will be referred to as protrusions in the following. The embossing can serve several purposes. It can be used to provide a stiffening effect, to impress an esthetic pattern or to provide a ply bonding between a multitude of individual plies in the web.
The embossing devices can comprise one flat steel roll and one embossed positive (male) roll which is generally referred to as an SSE-system (single soft embossing). In this system the paper plies are embossed on a hard roll with hard protrusions and then led through a nip between the hard embossing roll and another hard roll with a flat surface. In this nip the plies are mechanically bonded to each other. A suitable material for the hard rolls and the protrusions are steel. Other embossing devices are the UNION-embossing or the nested embossing which are especially used to provide ply bonding between different tissue plies with or without the additional use of an adhesive. Beside the technical effect of the embossing, the esthetic effect increasingly gains importance. Consequently, there is a desire to produce different products with different embossing patterns. Moreover, it has to be distinguished between brand products and so-called private or white label products. Quality features like softness, or stability can usually only be recognised during use of such a product. Therefore, the provision a brand name or trademark on the products is often the only recognisable difference between brand products and white label products.
Since different products are manufactured on the same paper converting machines, it is necessary to changeover these machines to different products with different embossing designs. There is a constant desire to reduce the changeover times. These are usually determined by the required time to exchange the embossing rolls. The disassembly and assembly of a roll, the correct adjustment of the nip between the parallel rolls and the exact positioning of the embossing roll relative to the counter roll requires about one working day. This leads to a certain inflexibility of the production because a good production planning normally tries to avoid any unnecessary exchange of the rolls.
The positioning of two complete embossing stations with different embossing rolls which are positioned one after the other in the running direction of web to be processed might reduce the downtime during the exchange or the maintenance of an embossing roll. However, this is a very expensive and, moreover, space consuming solution. Due to the space restrictions it is in most cases not possible to integrate a further complete embossing station into an existing production line. DE 199 36 278 suggest to provide an embossing station with at least two embossing rolls positioned around a counter roll. Each embossing roll can be moved between an operating position and a resting position. In the resting position, there is a sufficient nip between the embossing roll and the counter roll so that, while the web is passing through, this nip is not activated.
EP 0 275 231 B1 describes an embossing station for paper products which can achieve more than one pattern on a web without exchange of embossing rolls. Based on the object to shorten the time for a change of the embossing pattern, it is suggested to use a foot-to-foot embossing between rolls which are separately driven at synchronized speed. By changing the degree of overlap between mutually facing pairs of protrusions which determines the embossed areas, the embossed areas and, therefore, the embossing pattern can be adjusted between a maximum overlap and a minimum overlap. The degree of overlap between the mutually facing pairs of protrusions can be adjusted by axial or radial displacement of at least one of the two rolls.
It is the object of the invention to provide an embossing device which minimizes the changeover time and, at the same time, provides a high flexibility in changing the embossing patterns.
The inventive embossing device has first embossing protrusions on the circumference of a first roll cooperating with a second roll to form a first nip with a first gap in-between. There are second embossing protrusions on the circumference or the first or second roll cooperating with the other first or second roll as a counter roll to form a second nip with a second gap in-between. Further, the embossing device comprises an adjusting means for selectively adjusting the second gap. This provides a high variability of the embossing device. As an example, the first protrusions could form an esthetic embossing pattern, whereas the second protrusions might add a certain brand name or trademark to the embossing pattern. The provision of the adjusting means makes it possible to selectively adjust the second gap, which is the gap between the brand name or trademark and the counter roll. Therefore, it is possible to fade out the provision of the trademark or brand name in the overall embossing pattern of the product. Another possibility is to selectively adjust the second gaps so that the impressing depths of the second protrusions leads only to the embossing of the brand name on one side of the product. This avoids the occurrence of a mirror-inverse impression of the embossing pattern on the backside of the product. The above given two examples only cover a part of the different possibilities of product variations which can be easily performed with the inventive device. Besides these examples all relating to the selective adjusting of the second gap independently of the first gap, there are also possibilities for completely switching between two different embossing patterns. In that case the adjusting means for selectively adjusting the second gap also influences the first gap.
According to a preferred embodiment of the invention, the first roll is made of steel and the second roll is made of rubber. According to an alternative preferred embodiment, the first and second roll are made of steel. In that case using an SSE-system, it is preferred to provide one flat steel roll and one embossed positive steel roll cooperating with each other. The inventive embossing device can also be of a union-type.
Preferably, the first and the second roll are synchronised relative to each other. This is important in all those configurations, where one of the rolls is not a flat roll.
Then the embossing device is only operable if the two cooperating rolls are in register so that the relative position of the protrusions and/or depressions on both rolls are always in a predetermined and controlled position relative to each other.
The synchronisation of the two rolls can be advantageously achieved by means of a gear box or a synchronous belt drive between both rolls.
According to a preferred embodiment of the invention the adjusting means comprises a means for displacing the first and second roll relative to each other in an axial direction. Such an axial displacement changes the relative position of the second protrusions and corresponding protrusions or depressions on the counter roll relative to each other so that the gap of the nip in the region of the second protrusions can be selectively adjusted.
The same can be achieved by displacing the first and second roll in a circumferential direction relative to each other. A displacement in a circumferential direction can be performed by turning one of the rolls by a certain amount whereas the other roll is held still. Another theoretical possibility is to run both rolls synchronously, i.e. with the same circumferential speed and, during a certain changeover time, to rotate one of the rolls with a higher speed to achieve a circumferential shift between both rolls. However, this theoretical possibility is difficult to achieve in practice because the web passing between the two rolls undergoes a shearing load due to the differences in the circumferential speed between both rolls during the changeover operation. This could lead to a tearing of the product web to be processed. Moreover, it should be kept in mind that according to this embodiment the circumference of the rolls is important because it should have an integer number of design lengths. Each design pattern has a specific length in the circumferential direction of the embossing cylinders. The easiest variant is that both rolls have to have the same circumference and an equal number of design lengths. If the rolls have different circumferences, they have a circumference corresponding to an integer number of design lengths along their circumference. In other words, the ratio of the circumference and the length of the design should be an integer for both embossing rolls. Only then the embossing pattern will be constantly repeated on the web of tissue paper.
According to a preferred embodiment of the invention, the second protrusions are placed on additionally provided embossing elements, the position of which is adjustable in a radial direction of the roll so that they are movable from one effective, embossing position into a retracted, non-effective position. This preferred embodiment has the advantage that the movement of the additionally provided embossing elements acting as signature elements can be performed during the normal operation of the embossing device. The changeover time is reduced to a minimum, namely the time required to move these embossing elements from the embossing position into the retracted position and vice versa. Moreover, this embodiment opens up the option to selectively move only some of the second protrusions at one time.
If such additionally provided embossing elements are used, it is preferred to provide them with rounded edges. This avoids the inadvertent embossing of sharp edges of these embossing elements into the web of tissue paper.
According to preferred embodiment, the position of the additionally provided embossing elements is adjustable by means of an oil pressure system. The use of a hydraulic system has the advantage that the embossing pressure can be easily adjusted. Other possibilities do adjust the position of the additionally provided embossing elements comprises spring loaded or electromagnetically operated embossing elements of the lying of slims (thin plates) under the embossing elements. The embossing elements can also be adjusted by a wedging action of underlying slims cooperating with a tapered surface of the embossing elements. A preferred embodiment for providing an oil pressure system comprises the attachment of the embossing elements to pistons which are movable within cylinders in fluid communication with lines for pressurised oil. Depending on the oil pressure in the cylinders, the pistons move the attached embossing elements into the embossing position or the retracted position.
According to a preferred embodiment, the oil pressure system further comprises biasing elements to bias the embossing elements into the retracted, non-effective position. These biasing elements can be realised in form of spring elements which have to be selected to hold the embossing elements in the retracted, non-effective position against the centrifugal force during rotation.
According to a further preferred embodiment of the invention, there is additionally provided a means to selectively hold some selected embossing elements in the effective position whereas the other embossing elements are in the non-effective position. This embodiment enables the use of different additional design elements or brands which are selectively added to a basic pattern.
Common to all above cited preferred embodiments is the possibility to provide at least one of the rolls with a certain bombage in order to compensate the sagging of the rolls due to their weight.